I just picked GW2 because MMO games are usually easier to pick up for beginners, I have heard from many WoW players that they didn't play any other non-casual games before it. Also I am currently playing GW2 so I would be willing to babysit him.
I am not trying to make him play games to "show the light" and make him a hardcore gamer. I just want a companion(online friends are the different thing) to play together, and as for the casual games... I would rather play alone than sit through the Farmville or Tetris session.
I'd say about all you can do really is show it to them sometime when they are visiting with you. Tell them maybe you'd like them just to see it and just hang out and talk while you show them some questing, combat, etc. It might get them interested or it might not. Also, they might not express any interest that day but they might a few days later. It would be worth mention especially, how it could be a fun thing to be online slaying monsters together in this world while hanging out together.
When my girls were much younger and the MMO Asheron's Call was new, they saw me playing it and laughed at old dad playing such a silly game on the computer. They thought it was foolish and quite funny that I'd be doing that. Within days, they became curious and wanted to fool with it themselves so I let them. To make a long story short, this cost me two new PCs (for them) and resulted in our first home network being setup so everyone could play. It was the only way I could get any time on my own computer! They had great fun with it that summer, often playing together but lost interest again when school started up and that was that. They've never played an MMO since and neither of them, now out of college plays computer or console games at all. But they did have some fun with them as kids.
All you can do is offer someone a little exposure to see if they become interested. I don't think it is good to put too much effort into persuading someone to do an activity you want to but which they really don't. You might get someone to do something that way but their heart won't be in it and it won't last.
I do hope you have some luck showing it off and maybe they will like to try it. I can understand how it would be a lot of fun to play with a good friend.